There are many professions and activities in which participants may be exposed to physical contact which may result in head injury. For example, athletes participating in contact sports, such as American football, are subject to exposure to concussions, hyperextension, whiplash-type head movement, and cervical compressive forces. Football players at positions such as interior lineman, for example, are subjected to physical contact on virtually every play which can force the player's head rapidly backward, to create a whiplash effect which can result in serious and disabling injury. Additionally, it has recently been noted that glancing blows, or hits not directly on center, lead to concussions as well as torsional neck injury. Moreover, persons involved in activities such as high speed vehicle test piloting and race car driving can also be exposed to hyperextension or whiplash-type injuries caused by high rates of acceleration and impact forces. Military personnel are likewise exposed to combat and training situations which place them at risk of head injury. For participants in these and other activities, protective headgear such as helmets is often standard equipment.
Most blows to the head of participants in such activities are not direct, on-axis impacts, which cause linear acceleration. Generally, the blows to the head are glancing blows, with the head of a participant twisting as a result. Recent studies have shown that concussions are likely most often caused by rotational acceleration of the head, i.e., combined linear and angular acceleration.
Currently, the solution to prevent or minimize these injuries has simply been to add more padding to existing helmets. Some designs have added “crumple zones” to the exterior of the helmet, or a padded cap. These existing designs do not prevent or minimize the effects of glancing or off-center impacts which result in rotational acceleration.